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Choice for Zigbee PAN coordinator

Posted: 03 Oct 2005 ?? ?Print Version ?Bookmark and Share

Keywords:ieee802.15.4? zigbee? network? freescale? rfd?

Over the last year, the IEEE 802.15.4 standard and Zigbee specification based on that standard has created a stir in the electronics world due to the benefits Zigbee offers vs. that of other wireless standards available today. Zigbee is cheap to implement, optimized for very low-power operation, more reliable and portable. One of the key challenges in implementing a Zigbee network is developing an infrastructure, especially in some of its more complex application areas. Take for example one of the key growth areas for Zigbee, the industrial market, where typical installations may require the deployment of around 5,000 802.15.4 nodes, according to In-Stat.

In the industrial market, Zigbee networks will have to cover larger areas. For example, large offices or manufacturing sites will be deployed in different applications, ranging from heating, ventilation and airconditioning (HVAC) control to security and access control systems!both of these factors increase the complexity and, in turn, the number of nodes deployed. To enable the implementation of these systems across larger sites will require multiple layers of networks containing full-function devices (ffd) that communicate with multiple Zigbee nodes and reduced-function devices (RFD) that perform usually only point-to-point transmissions.

To organize such a vast number of wireless network nodes requires the creation of a tree-type structure to pass the information from different Zigbee networks back to a central control point, from where the systems can be monitored and controlled via a personal area network (pan) coordinator. The PAN coordinator is the focal point of an entire Zigbee network. The figure shows a simple Zigbee network that could be implemented in any building or industrial site to carry out HVAC control, security/access control and fire detection!typical applications in which Zigbee is expected to be implemented. All communication is done via the Zigbee network.

Typically, lower-level FFDs and RFDs will be driven by an mcu interfaced to a Zigbee transceiver over a QSPI interface. The MCU chosen will depend on whether it is acting as an FFD with further Zigbee network layers beneath it. A basic RFD will usually be driven by an 8bit MCU, but depending on the complexity of the FFD and the network it is connected to, this could be either an 8bit, 16bit or low-end 32bit-based MCU.

Central control point

The PAN coordinator is a critical point in a robust Zigbee network, as it is responsible for coordinating the entire network and communicating back to the central control point. The key requirements for a PAN coordinator are:

? Drive communications for the entire PAN!Due to the increased amount of communications traffic on a large PAN, higher bandwidth through a PAN coordinator is required.

? Map the entire Zigbee PAN!The PAN coordinator has to store a map of the entire network and identify which nodes within the network are FFDs or RFDs and the functions of each part. For larger, complex industrial systems, more memory to store this map is needed.

? Have the ability to dynamically establish links with new nodes on network!In large systems, it is highly likely that at some point in the lifecycle, new nodes will need to be integrated into the system. The PAN coordinator needs to establish links with those new nodes at any point in the network. For the PAN coordinator to perform this task effectively, more local program memory is required!hence the ability for the PAN coordinator to address that memory.

In larger, more complex systems (e.g. a manufacturing site), it is very likely that the central control point may not be within range of the Zigbee network; it may even be in a separate building. Thus, it is likely that the PAN coordinator will communicate with the central control point via a wired connection. With the increasing adoption of Ethernet in the industrial market, it would seem to be the most likely choice in most instances. The implementation of Ethernet in the system has two potential effects on a network design:

? Considerations need to be made with regard to the processor bandwidth required to handle the Ethernet interface;

? To drive the Ethernet interface, the network will require the relevant low-level drivers and protocol stacks, thus increasing the amount of program memory required by the PAN controller in the system.

Good memory

One of the key benefits of implementing a microprocessor-based solution is that many MCUs may not offer enough integrated memory to drive all the required low-level drivers, program memory and protocol stacks, as well as potentially run an RTOS. All this functionality could result in a system requiring anywhere between 1MB and 4MB of memory, more than what most standard MCUs offer and some 8/16bit MCUs can address. Rather, the MCF5208 has an integrated DDR SDRAM controller that is backward-compatible with SDR DRAM, giving developers a choice of DRAM configurations, depending on their system requirements.

Additionally, the external bus can interface with flash, EEPROM, ROM and SRAM, again providing the developer with the flexibility to integrate the memory they feel best suits their system requirements. For optimal execution of code, there is 8KB of unified cache memory and 16KB SRAM integrated on-chip. Finally, the MCF5208 has been designed specifically for low-power implementation!a key requirement in Zigbee applications.

- Gordon Padkin, Product Marketer

32bit Standard Products Operation, Transportation and Standard Products Group

Freescale Semiconductor Inc.




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